If enough people look long enough for UFOs, somebody will see one and tell their friends about it. The military can then pretend that they have man-made secret aircraft based on UFO technology that they reverse engineered. That's in the hope that their enemies will believe them and be too scared to give them aggro. Some are probably stupid enough to fall for it. After all, you can see how many people believe in Aliens.
You have to ask yourself why do the BBC keep recycling the old videos "leaked" from the military, who say that they have no idea what the thing on the video is, when they've all been debunked and explained by rank amateurs on youtube.
This was the last one, though the BBC have repeated other previous ones since. debunking the green triangles starts about half-way through:
It's a bit like climate change. Any climatologist knows about Milankovich Cycles and other climate cycles. They're very consistent, and they show that the CO2 levels match exactly with temperature. Some years ago, politicians and other tyrannists figured out that the cycles were all ascending, so both temperature and CO2 were going up, and they could blame it on normal people and use it to leverage their tyrrany. People are very religious, so they accept these new religions extremely easily. You could say that they're gullible, but pride prevents people from accepting that.